The present invention relates to system or process for controlling an idle speed of an internal combustion engine substantially constant.
A conventional idle speed control system is arranged to sense an engine speed directly and controls an air quantity (the opening degree of an auxiliary air valve) in a feedback control mode so as to reduce a deviation from a desired idle speed.
When, however, an external load is added, for example, by turning on an air conditioner or shifting an automatic transmission from the neutral position to the D range, the engine speed falls instantaneously. The above-mentioned feedback system is slow in response and ineffectual in preventing an abrupt decrease of the idle speed due to the addition of the external load.
An idle speed control system as disclosed in Japanese Patent Provisional Publication No. 60(1985)-240843 is arranged to detect an addition of an external load and to increase the fuel injection quantity upon detection of the addition of the external load. When the external load is removed, this system decreases the fuel injection quantity.
However, in the system in which the decrease of the fuel injection quantity and the removal of the load are performed simultaneously, the air fuel mixture tends to become too lean. Specifically, in a lean burn engine designed to run on a significantly low air/fuel ratio to improve the fuel consumption, the air/fuel ratio might reach the lean limit by the decrease of the fuel injection quantity, quantity, resulting in poor combustion and unpleasant drive feeling.
On the other hand, the simultaneous operation of increasing the fuel injection quantity and adding the external load makes the air fuel mixture richer, and deteriorates the fuel consumption and exhaust performance.